Semantic Web-Based Information Systems: State-of-the-Art ...
Semantic Web-Based Information Systems: State-of-the-Art ...
Semantic Web-Based Information Systems: State-of-the-Art ...
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Specific Reasoning as Theories<br />
Query ng <strong>the</strong> <strong>Web</strong> Recons dered 0<br />
Many practical applications require special forms <strong>of</strong> reasoning; for instance, efficient<br />
equality reasoning is <strong>of</strong>ten performed using <strong>the</strong> so-called paramodulation<br />
rule instead <strong>of</strong> <strong>the</strong> equality axioms (transitivity, substitution, and symmetry). Also,<br />
temporal data might require conversions between different time zones and/or calendar<br />
systems that are expressed in a simpler format and more efficiently performed using<br />
arithmetic instead <strong>of</strong> logical axioms. Finally, reasoning with intervals <strong>of</strong> possible<br />
values instead <strong>of</strong> exact values (e.g., for appointment scheduling) is conveniently<br />
expressed and efficiently performed with constraint programming.<br />
For this reason, it is desirable that a query language for <strong>the</strong> (conventional and<br />
<strong>Semantic</strong>) <strong>Web</strong> can be extended with so-called “<strong>the</strong>ories” implementing specific<br />
forms <strong>of</strong> reasoning.<br />
Such “<strong>the</strong>ory extensions” can be realized in two manners:<br />
• First, a <strong>the</strong>ory can be implemented as an extension <strong>of</strong> <strong>the</strong> runtime system <strong>of</strong> <strong>the</strong><br />
query language with additional language constructs for using <strong>the</strong> extension.<br />
• Second, a <strong>the</strong>ory can be implemented using <strong>the</strong> query language itself and made<br />
available to users <strong>of</strong> this query language through program libraries. In this<br />
case, <strong>the</strong>ories are implemented by rules and queries. <strong>Based</strong> upon <strong>the</strong> XML<br />
syntax <strong>of</strong> <strong>the</strong> query language (Section 2.12), for example, such rule bases can<br />
<strong>the</strong>n be queried using <strong>the</strong> query language itself and maintained and updated<br />
by a reactive language such as XChange (Bry & Ptrânjan, 2005).<br />
Querying Ontologies and Ontology-Aware Querying<br />
In a <strong>Semantic</strong> <strong>Web</strong> context, ontologies can be used in several alternative ways. First,<br />
<strong>the</strong>y can be dealt with by a specialized ontology reasoner (<strong>the</strong> main disadvantage<br />
being <strong>the</strong> impossibility <strong>of</strong> adding new domain-specific constructs). Second, <strong>the</strong>y can<br />
be regarded as descriptions to be used by a set <strong>of</strong> rules implementing <strong>the</strong> <strong>Semantic</strong>s<br />
<strong>of</strong> <strong>the</strong> constructs employed by <strong>the</strong> ontology. (This is similar to a meta-interpreter and<br />
may be slow.) Alternatively, <strong>the</strong> ontology may be “compiled” to a set <strong>of</strong> rules.<br />
As discussed in <strong>the</strong> previous point, <strong>the</strong> query language should allow for both approaches:<br />
extending <strong>the</strong> query language by specific <strong>the</strong>ory reasoners for a certain<br />
ontology language (e.g., OWL-DL) as well as <strong>the</strong> ability to use rules written in <strong>the</strong><br />
query language as means for implementing (at least certain aspects) <strong>of</strong> an ontology<br />
language. Examples for such aspects are <strong>the</strong> transitivity <strong>of</strong> <strong>the</strong> subsumption<br />
hierarchy represented in many ontologies or <strong>the</strong> type inference based on domain<br />
and range restrictions <strong>of</strong> properties.<br />
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